1. Field of Application
The present invention relates to an electrical power conversion apparatus which incorporates a transformer, such as a DC-DC converter which converts an input DC voltage to a different value of DC voltage.
2. Background Technology
An electric power converter which operates as a DC-DC converter is described for example in Japanese patent No. 3615004, referred to in the following as document D1. The power converter of document D1, e.g., as shown in FIG. 2 thereof, includes a transformer having primary and secondary windings, a converter circuit which receives a DC supply voltage from a main battery, a rectification and smoothing circuit, and a control circuit. The output DC voltage from the apparatus is supplied to charge a secondary battery. The converter circuit includes high-side and low-side switching elements, which are controlled by signals from the control circuit to apply alternating-polarity voltage pulses to the primary winding of the transformer, as an AC input voltage. A stepped-down AC voltage is thereby produced from the secondary winding of the transformer due to AC current flow in the primary winding, and is converted to the output DC voltage by the rectification and smoothing circuit. The control circuit controls the voltage pulses applies to the primary winding (e.g., by adjusting the pulse width) in accordance with the output DC voltage.
A problem which arises with such a type of circuit is that, if there are deviations between the operating characteristics of certain circuit elements in the input circuit such as semiconductor switching devices, due to manufacturing variations, the AC current flowing in the primary winding of the transformer may contain a DC component which is superimposed on the AC current. A DC flux bias will thereby be produced in the magnet flux of the transformer. If the current contains a positive-polarity DC component for example, then each positive-polarity current pulse is increased in amplitude by the value of the DC component, while similarly each negative-polarity current pulse is decreased in amplitude by the same amount.
It would be possible to suppress such a DC component by applying suitable negative-feedback current control. That is, in the case of a positive-polarity DC component occurring for example, each positive-polarity voltage pulse could be determined (e.g., pulse width or amplitude decreased) based on a precedingly detected value of positive-polarity current and each negative-polarity voltage pulse correspondingly determined based on a precedingly detected value of negative-polarity current flow. The effect of the DC component in producing a DC flux bias could thereby be gradually suppressed.
However when digital control is applied, the following problem arises with the prior art. In general, each voltage pulse applied to the primary winding is determined based on a value of current flow in the primary winding which was detected during the immediately preceding voltage pulse. In that case, again assuming a positive-polarity DC component, each positive-polarity voltage pulse will be controlled based on a preceding negative-polarity current pulse, and each negative-polarity voltage pulse will be controlled based on a preceding positive-polarity current pulse. When feedback control is performed in such a condition, the DC component of the primary winding current (and hence the DC flux bias) will become increased rather than decreased.